Book Description

This dissertation, "Time-resolved Resonance Raman Investigation of Selected Para-substituted Phenylnitrenium Ions and the 2-fluorenylnitrenium Ion Reaction With Guanosine" by Pik-ying, Chan, 陳碧瑩, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled TIME-RESOLVED RESONANCE RAMAN INVESTIGATION OF SELECTED PARA-SUBSTITUTED PHENYLNITRENIUM IONS AND THE 2-FLUORENYLNITRENIUM ION REACTION WITH GUANOSINE Submitted by Pik Ying CHAN for the degree of Doctor of Philosophy at The University of Hong Kong in August 2005 Arylnitrenium ions (ArNR ) are believed to play an important role in the carcinogenic reactions of aromatic amines with DNA. Arylnitrenium ions are very short-lived species and difficult to study. However, photochemical methods have recently been developed to study them. There is currently only limited structural information available for arylnitrenium ions and their reaction intermediates. This thesis presents the use of time-resolved resonance Raman (TR ) and density functional theory (DFT) to study selected arylnitrenium ions and their reactions with a guanine derivative. This work provides new and valuable structural and chemical reactivity information about the selected arylnitrenium ions investigated in this thesis. The first time-resolved resonance Raman spectra for the 4-methoxyphenylnitrenium, 4-ethoxyphenylnitrenium and 4-acetamidophenylnitrenium ions are reported. The experimental Raman bands observed in conjunction with the DFT calculation results indicates these arylnitrenium ions are singlet state species. These para-substituted phenylnitrenium ions were found to have significant iminocyclohexadienyl character to a degree very similar to that previously found for the 2-fluorenylnitrenium ion. The 4-methoxyphenylnitrenium and 4-ethoxyphenylnitrenium ions also have significant oxocarbocation character while the 4-acetamidophenylnitrenium ion has noticeable charge on both the acetamido and nitrenium moieties. Their structure and properties are compared to those of other arylnitrenium ions. The chemical reactivity and selectivity of the 4-acetamidophenylnitrenium ions are compared to the two alkoxy-substituted phenylnitrenium ions and other para-substituted phenylnitrenium ions. A picosecond Kerr-gated time-resolved resonance Raman examination of the 2-fluorenylnitrene and 4-methoxyphenylnitrene reactions with water to form the singlet 2-fluorenylnitrenium and 4-methoxyphenylnitrenium ions, respectively, is given. Photolysis of 2-fluorenyl azide and 4-methoxyphenyl azide in mixed water/acetonitrile solvents both led to formation of transient species within a few picoseconds, and these were tentatively assigned to the 2-fluorenylnitrene and 4-methoxyphenylnitrene species, respectively. These arylnitrenes are observed to directly react with water to produce their respective singlet arylnitrenium ions. The decay of the 4-methoxyphenylnitrene species and its accompanying growth of the 4-methoxyphenylnitrenium ion were found to be much faster than the decay of the 2-fluorenylnitrene and its accompanying growth of its 2-fluorenylnitrenium ion, indicating these two different arylnitrenes have noticeably different chemical reactivity toward water. Time-resolved resonance Raman spectroscopy was also applied to directly observe the reaction of the 2-fluorenylnitrenium ion with guanosine to produce a "C8 intermediate" species. This is the first time-resolved vibrational spectroscopic characterization of an arylnitrenium ion reaction with a guanine derivative and of a "C8 intermediate"